The application of an electrical current to certain semiconductor diodes doped so as to contain a pn-junction produces electromagnetic radiation. The energy supplied is converted into the radiant energy emitted to the outside and into heat energy consumed in heating the device itself. One of the main heat sources is non-radiative recombination, i.e. the process in which the energy of the recombining electron-hole pair is transformed into phonons. Another heating effect is reabsorption of the radiation generated in the active region. These and other heating effects not only reduce the efficiency of the device but also, by increasing the temperature inside the device, adversely affect the properties of the emitted radiation beam thereby deteriorating the operating characteristics of the device. The parameters of the radiation beam such as the radiation power, the near-field and far-field patterns, and the spectral characteristics strongly depend upon the heat generation process and on the efficiency with which this heat is dissipated. In addition, the electrical characteristics of the device such as the current-voltage characteristic curve are negatively affected, the useable output power is degraded and reliability/lifetime is reduced by increases in the device junction temperature.
Multiple laser diodes are fabricated onto a single substrate into a linear laser diode array. To control the type and number of spatial modes the laser diode will emit, the electrical current and lasing emission are confined to a narrow stripe geometry. Multiple stripes are combined into a linear array, sometimes called a bar. In order to achieve high radiation intensity levels it is desirable to stack these bars into two dimensional arrays. This becomes important, for example, in the pumping of solid state lasers. In order to obtain high optical power levels, it is desirable to stack these linear diode array bars in close proximity into two dimensional array structures. Such structures, however, have to possess means for waste heat removal, mechanical integrity and electrical interconnection. The problem that this invention addresses is that of obtaining the maximum stacking density of linear laser diode array bars while at the same time removing sufficient waste heat such that the operation of the laser diodes is not adversely affected.
It is also desirable that the temperature along each bar be uniform so that each stripe of the bar can be operated phase locked. It is also desirable that the semiconductor bar and the heatsink onto which it is mounted have thermal coefficients of expansion which are closely matched to minimize thermal stress. Thermal stresses cause an undesirable shift in wavelength of the emitted radiation and can damage the linear laser diode array bar.
Among other applications, laser diodes provide a near ideal radiation source for pumping solid-state lasers. They combine very high brightness, high efficiency, monochromaticity and compact size. One of the most important problems in semiconductor laser design is realizing the potential of laser diodes and achieving maximum cw output power without locally overheating in the active regions.
Cooling techniques to avoid this problem have included forced air or liquid cooled radiators, micro-channel heat sinks through which cooling liquids are forced, copper heatsinks, diamond heatsinks, diamond coated copper heatsinks, silicon heatsinks, thermoelectric coolers, impingement coolers, cryogenic refrigerators and liquid nitrogen dewars. Examples of placing thermally conductive materials between laser diode bars in a two dimensional array can be found in U.S. Pat. Nos. 4,716,568 to Scifres and Harnagle, 4,454,602 to Smith and 4,393,393 to Allen. Each of these mounts a single laser diode array bar per heatsink due to the limited heat carrying capacity of the materials used. Other approaches separate the laser diodes so that they can be cooled individually and then attempt to combine their radiation via fiber optic bundles.
Each of these approaches has its limitations and none has been able to remove sufficient waste heat from between laser diode arrays to permit true CW (continuous) operation, although "quasi-CW" operation is often claimed, the term "quasi" meaning that the repetition rate is high enough to look continuous to the human eye. This thermal problem is the most serious problem facing the application of laser diode arrays to a wide variety of high-intensity applications.
These inventions are dependent upon using heatsink metals with a high thermal conductivity, e.g. copper and aluminium. Unfortunately, metals with a high thermal conductivity also have high thermal coefficients of expansion (CTE) relative to that of the laser diode substrate. This difference in CTE between substrate and heatsink results in thermal stresses which degrade the optical properties and induce damage into the laser diode substrate. What is required is a high thermal conductivity heatsink with a CTE closely matching the laser diode materials.
In U.S. Pat. No. 4,791,634 to Miyake heat is conducted away from between a two dimensional array of laser diode bars by interleaved heatsink strips, similar to those described under the patents referenced above, that conduct heat away from the laser diode array bars to the evaporator of a distant heatpipe, i.e. the laser diodes are not mounted directly onto the heatpipe.
Heatpipes are devices that are able to carry very large heatfluxes by utilizing the heat of vaporization of a fluid. A specified amount of the fluid is sealed within an elongated enclosure with the heat flux entering at one end, termed the evaporator, causing evaporation of the fluid. The resulting vapor phase travels the length of the enclosure and condenses on the cooler opposite end of the enclosure where the heat of condensation is removed to the outside. The liquid phase is returned to the evaporator end by capillary action via either arteries contained within the enclosure walls or via a fine mesh wick structure lining the walls of the enclosure.
Cotter's original proposal for micro-heatpipes involved simple channels formed by anisotropic etchants. Although others have subsequently improved upon Cotter's fabrication methodology, none have been able to fabricate the kinds of complex structures required to obtain good heat-transfer performance in sizes small enough to place between laser diode array bars. "Micro" was defined by Cotter to be a heat pipe so small that the mean curvature of the vapor-liquid interface is comparable in magnitude to the reciprocal of the hydraulic radius of the total flow channel. In practice, this translates into an enclosure with an internal vapor space channel with an approximate diameter of 100 to 500 micro-meters.
Although the concept of micro-heatpipes has been discussed in the literature and many attempts have been made to fabricate them, no one has been able to fabricate them with sufficiently small dimensions necessary to classify it as "micro" and with the cross-sectional design necessary to enable it to remove sufficient heat from between laser diode arrays to solve the problem described above. In fact, most if not all designs remain crude multi-sided channels in which the capillary pumping is done in the interstices, e.g. in the corners of triangles, squares or distorted squares.
These designs suffer from many limitations, the most serious being entrainment in which droplets of the unprotected liquid in the corners of the structure are ripped from the liquid surface and returned to the condenser end causing the evaporator end of the micro-heatpipe to dryout and fail. It is important to protect the liquid phase of the fluid by artery design, for example by protecting the artery with a re-entrant groove. This is difficult to do in a heatpipe that is small enough in diameter to be placed between two laser diode arrays so that high optical power levels are achieved.